Many cancer patients take dietary supplements in hopes of keeping their disease at bay, but British researchers say there's little evidence it will pay off.
In fact, they add, supplements may not only be ineffective, but harmful as well.
"We found 1 in 5 people who had been treated for cancer mistakenly thought that taking vitamins or other supplements would help reduce the risk of their cancer coming back," said Rana Conway, a research fellow at University College London.
Dietary supplements aren't recommended for that purpose, she said, as they have not been shown to help.
People who have had cancer are advised to eat a healthy diet with plenty of fruit and veggies and to avoid high-calorie foods, Conway said. Staying away from alcohol and being physically active are also beneficial, she added.
It's important for cancer patients who are considering dietary supplements to speak with their doctor first.
"There is no evidence that self-prescribed supplements reduce the risks of cancer coming back, and they could interfere with treatment," Conway said. "Most of us find healthy eating and exercise advice more difficult to stick to, but the evidence shows it offers real benefits."
For the study, Conway's team collected data on more than 1,000 adults who had been diagnosed with breast, prostate or colon cancer and were part of a British research trial. Participants completed a questionnaire about their diets and any supplements they took.
The most common ones they used were fish oil, taken by 13% of study participants, and calcium (with or without vitamin D), taken by 15% of those with breast cancer.
In all, 40% of participants were taking dietary supplements, and 19% believed they reduced the odds of their cancer returning, the study found.
Among women, those who ate recommended amounts of fruits and vegetables and who thought supplements would help keep cancer at bay were more likely to use them, researchers said.
Conway's team also found that patients who were obese were less likely to use supplements.
Marji McCullough, senior scientific director for epidemiology research and population science at the American Cancer Society, reviewed the findings.
"It was interesting to learn that such a high proportion of cancer patients think that supplements will lower their risk of recurrence," she said.
McCullough suspects folks who take supplements have been influenced by what they see on the Internet and in ads, rather than their doctors.
"I think people want to do what they can to improve their prognosis," she said.
Along with the supplements cited in the study, cancer patients are also taking multivitamins, thymine, vitamin D, B vitamins and herbal supplements, McCullough said.
But some supplements are known to affect cancer treatments, she warned.
For example, folate (a B vitamin) may interfere with certain chemotherapy drugs, and antioxidants like vitamin C and D might make cancer treatments less effective. Vitamin A could contribute to liver toxicity, McCullough said.
"These are some of the ones we know about," she said. "There's a lot that we don't know about."
Patients who use supplements should tell their doctor which ones they take, and doctors should ask patients if they use supplements, McCullough said. Getting the advice of an oncology dietitian is also a good idea, she said.
These dietitians, who specialize in helping cancer patients, can assess whether a supplement is needed to treat low levels of some vitamins and also help patients develop a healthy diet plan, McCullough said.
"Many cancer patients have unique needs as they go through treatment and beyond treatment," she said. "The American Cancer study recommends that cancer patients should always discuss dietary supplement use with their doctor -- that's the bottom line."
The findings were published online Dec. 20 in the journal Cancer.
For more on cancer and dietary supplements, visit the American Cancer Society.
SOURCES: Rana Conway, PhD, research fellow, behavioral science and health, University College London; Marji McCullough, ScD, RD, senior scientific director, epidemiology research, population science, American Cancer Society; Cancer, Dec. 20, 2021, online